A typical screening machine has a deck or screen with openings which are sized to pass particles having a maximum dimension smaller than the openings. Although such decks are widely used for making separations according to size, they are relatively inefficient for separating chips such as woodchips wherein the critical dimension is the thickness of the chip rather than the dimensions of its face. For example, woodchips of the type used in the manufacture of pulp are cut in roughly plate-like form, approximately square to elongated rectangular, having facial dimensions which are several times chip thickness. At the present time the optimal or desired dimensions are approximately 1/2 to 11/2 inches in length and width, and about 3/8 inch (about 8 millimeters) in thickness. The importance of thickness as the critical dimension arises from the fact that thickness, rather than length or width, limits the penetration of digesting chemicals into the center of the chip.
Chips as they come from commercial chipmaking processes comprise an almost continuous spectrum of sizes and thicknesses. "Accepts," that is, chips which are within the proper thickness range, must be separated from "fines," which are below the desired size range, and also from "overthicks" or "overs" which have a thickness greater than a predetermined maximum limit for the particular digestion process. (While there is sometimes a very rough correlation between chip facial dimensions and chip thickness, a separation according to length or width does not generally provide an acceptable separation according to thickness.) Conventional decks are quite efficient in removing fines, but they are much less efficient in making accurate separations of overs according to thickness and without regard to length and width. This is because conventional decks tend to pass chips according to surface length and width rather than thickness, which is typically less than length and/or width. The problem of separating chips according to thickness is much more difficult than that of separating chips according to facial dimension, especially where the chips are coarse in thickness and tend to be tapered. Screen blinding becomes a severe problem; and no matter how efficient, a screen which blinds in a short period of time is impractical for large scale commercial use, because of frequent shut-down required to clear chips lodged in the deck openings.